fix: use text/html+mcp mimeType format

Change from text/vnd.mcp.ui+html to text/html+mcp.

Rationale:
- It's an unregistered subtype, allowed for private use
- The +suffix form indicates MCP-specific semantics (postMessage protocol)
- Content is still valid HTML and can be rendered as-such
- Following RFC 6838, +mcp could be registered as an official suffix

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

examples/simple-server/server.ts

@@ -47,7 +47,7 @@ const getServer = async () => {
     uri: "ui://vanilla",
     title: "Vanilla UI Template",
     description: "A simple vanilla JS UI",
-    mimeType: "text/vnd.mcp.ui+html",
+    mimeType: "text/html+mcp",
   };
 
   server.registerResource(
@@ -89,7 +89,7 @@ const getServer = async () => {
     uri: "ui://react",
     title: "React UI Template",
     description: "A React-based UI",
-    mimeType: "text/vnd.mcp.ui+html",
+    mimeType: "text/html+mcp",
   };
 
   server.registerResource(